<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <style>
    body,
    ul {
      margin: 0;
      padding: 0;
    }
    body {
      width: 1920px;
      height: 1080px;
      box-sizing: border-box;
      border: 3px solid red;
    }
    ul {
      width: 100%;
      height: 100%;
      list-style: none;

      display: flex;
      flex-direction: row;
      flex-wrap: wrap;
    }
    li {
      width: 33.333%;
      height: 50%;
      box-sizing: border-box;
      border: 2px solid green;
      font-size: 30px;
    }
  </style>
  <body>
    <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
    </ul>
  </body>
</html>
